Materials possessing an optimal combination of high electrical conductivity, optical transparency and flexibility are components that are extremely important to the development of many high value-added fields, and especially any field related to “large area” electronics, i.e. electronics produced by printing techniques and at low temperatures.
The transparent electrodes usually used are manufactured from metal oxides (indium tin oxide, fluorine-doped tin oxide, etc.) and are not flexible. They lose their conductive performances when they are bent with small radii of curvature (for example smaller than 8 mm, and especially repeatedly).
Recent advances in the field of nanotechnology have made it possible today to foresee credible alternative ways of producing thin transparent heating films. Specifically, a number of transparent electrodes based on carbon nanotubes, graphene, polymers (such as PEDOT:PSS) or metal nanowires (metal=essentially silver) have been reported. These transparent electrodes may be produced on transparent plastic films and therefore identified as flexible transparent electrodes.
Up to now, the best performances have been obtained with metal nanowires. The transparent conductive system is obtained by manufacturing a percolating network of metal nanowires on a surface. These networks are obtained by depositing a suspension of nanowires in a solvent (water, methanol, isopropanol, etc.). The networks of silver nanowires have a low sheet resistance at 90% transmittance (measured at 550 nm), typically lower than 60 ohms/square (Yu, Zhang et al. 2011, “Highly Flexible Silver Nanowire Electrodes for Shape-Memory Polymer Light-Emitting Diodes” Advanced Materials 23(5): 664-668).
For example, for an optoelectronic device application, a low electrical resistance facilitates the passage of current. By way of example, this may improve the performances of photovoltaic cells (efficiency) or of display screens based on OLEDs (organic light-emitting diodes) or PLEDs (polymer light-emitting diodes) (higher luminous efficacy).
In another example, that of flexible transparent heating films, decreasing resistance allows heating performances to be improved, because the thermal power dissipated by the heating film is proportional to V2/R (Joule heating). Here V is the DC (direct current) voltage applied across the terminals of the heating film and R is the resistance of the device from one terminal to the other.
It would be desirable to improve said existing performances further.